This invention relates to a method of case hardening and is more particularly concerned with a method of case hardening an article formed of titanium, zirconium or an alloy of titanium and/or zirconium.
In engineering applications, when a surface is subjected to a high contact load by another body, internal stresses are developed below the surface, the so-called Hertzian stresses. These stresses reach a maximum at a certain depth below the surface. Consequently, in order to withstand such stresses, it is necessary for a case-hardened layer to provide increased strength (and therefore hardness) down to at least that depth. At the same time, it is desirable to avoid excessive hardness at the surface itself as this could cause embrittlement. To reconcile these requirements, it is generally preferred to produce a hardness profile, in the direction normal to the surface, which has a sigmoid shape (see, for example, the OD curve in accompanying FIG. 2), consisting of a region of relatively high hardness maintained to a certain depth below the surface before dropping more steeply and then gradually to the hardness of the untreated core material.
Both theoretical and experimental work has shown that significant improvements in the load-bearing capacity of a hard coating/sub structure system can be achieved provided that, in addition to a high interfacial adhesion strength, the substrate can firmly withstand the applied load without appreciable plastic deformation. This means that deep case surface engineering processes are beneficial for subsequent hard thin coatings on titanium alloys in view of their inherent low yield strengths and low elastic moduli. However, most titanium alloys, unlike ferrous materials, cannot be hardened to a great extent by conventional surface engineering techniques since there is no hardening reaction in titanium alloys comparable to the martensite transformation in ferrous materials. Notwithstanding the fact that titanium alloys can be deeply hardened by electron beam surface alloying, it is still difficult in practice to achieve controlled reproducibility of composition in the alloyed surface layer. Oxidising titanium alloys at a high oxidation temperature for an extended period of time can also produce a deep hardened case. However, simple oxidation at higher temperatures (greater than 700xc2x0 C.) is prone to the formation of severe scaling, resulting in a crumbly surface oxide layer. The present invention relates to a method which avoids this by oxidation treatment at an elevated temperature effected for a relatively short period of time, followed by a subsequent heat treatment operation.
A method of surface hardening titanium by oxygen is disclosed by A. Takamura (Trans JIM, 1962, Vol. 3, pages 10-14). In one of the methods disclosed by Takamura, samples of commercial titanium are annealed, polished and degreased and are then oxidised in dry oxygen at 850xc2x0 C. for 1 or 1.5 hours. A thin oxide scale is formed on the surface of the samples. Then, the thus-oxidised samples are subjected to a diffusion treatment at 850xc2x0 C. for 24 hours in argon so as to cause oxygen to diffuse into the sample. In other methods disclosed by Takamura, the oxidised samples are diffusion treated first in argon and then in nitrogen or are diffusion treated in nitrogen. In no case, however, is the desirable sigmoid-shaped hardness profile achieved.
It is an object of the present invention to provide a process which is more capable of achieving the desirable sigmoid-shaped hardness profile than the last-mentioned publication.
According to a first aspect of the present invention, there is provided a method of case hardening an article formed of titanium, zirconium or an alloy of titanium and/or zirconium, said method comprising the steps of (a) heat-treating the article formed of titanium, zirconium or alloy of titanium and/or zirconium in an oxidising atmosphere containing both oxygen and nitrogen at a temperature in the range of 700 to 1000xc2x0 C. so as to form an oxide layer on the article; and (b) further heat-treating the article in a vacuum or in a neutral or an inert atmosphere at a temperature in the range of 700 to 1000xc2x0 C. so as to cause oxygen from the oxide layer to diffuse into the article.
According to a second aspect of the present invention, there is provided a method of case hardening an article formed of titanium, zirconium or an alloy of titanium and/or zirconium, said method comprising the steps of (a) heat-treating the article formed of titanium, zirconium or alloy of titanium and/or zirconium in an oxidising atmosphere at a temperature in the range of 700 to 1000xc2x0 C. so as to form an oxide layer on the article; and (b) further heat-treating the article in a vacuum or in a neutral or an inert atmosphere at a temperature in the range of 700 to 1000xc2x0 C. so as to cause oxygen from the oxide layer to diffuse into the article whereby to produce a sigmoid-shaped hardness profile.
The time for heat-treatment in step (a) is relatively short and depends, inter alia, upon the nature of the oxidising medium and the intended use of the article. Typically, the time may be, for example, from 0.1 to 1 hour, preferably 0.3 to 0.6 hour.
The heat-treatment in step (a) is conveniently effected at atmospheric pressure.
Steps (a) and (b) may be repeated at least once.
In the method according to said second aspect of the present invention, the oxidising atmosphere in step (a) preferably comprises oxygen as well as nitrogen, as this improves the adhesion of the predominantly oxide scale thus formed.
In the first and second aspects of the present invention, the oxidising atmosphere in step (a) is preferably air. The temperature in step (a) is preferably 700 to 900xc2x0 C., more preferably 800 to 900xc2x0 C., and most preferably about 850xc2x0 C.
The temperature in step (b) is preferably 700 to 900xc2x0 C., more preferably about 800 to 900xc2x0 C., and most preferably about 850xc2x0 C. It is most preferred to effect treatment step (b) in a vacuum, in which case the pressure is preferably not more than 1.3xc3x9710xe2x88x922 Pa(1xc3x9710xe2x88x924 Torr) Pa, and is conveniently about 1.3xc3x9710xe2x88x924 Pa (1xc3x9710xe2x88x926 Torr). The use of a vacuum is much preferred because it reduces the risk of unwanted contaminants being accidently introduced into the surface of the article during step (b).
In particular, it is important to prevent gaseous oxygen from reaching the solid surface during step (b) where it may dissolve or react so as to cause excessive hardness and potential embrittlement. Where the heat treatment in step (b) is effected in an inert or neutral atmosphere, any non-oxidising and non-reducing atmosphere may be employed, such as argon or other inert gas, provided that it contains no or only a low partial pressure of oxygen.
The time required for the heat treatment in step (b) is typically in the range of 10 to 50 hours and may conveniently be about 20 to 30 hours.
It is within the scope of the present invention to follow the treatment steps (a) and (b) with any of a variety of subsequent treatments or processes to reduce friction. In particular, it is within the scope of the present invention to follow the method of the present invention with the treatment method disclosed in our copending PCT Publication No. WO98/02595 for improving the tribological behaviour of a titanium or titanium alloy article. Such process basically involves the gaseous oxidation of the article at a temperature in the range of 500 to 725xc2x0 C. for 5.0 to 100 hours, the temperature and time being selected such as to produce an adherent and essentially pore-free surface compound layer containing at least 50% by weight of oxides of titanium having a rutile structure and thickness of 0.2 to 2 xcexcm on a solid solution-strengthened diffusion zone where the diffusing element is oxygen and the diffusion zone has a depth of 5 to 50 xcexcm.
The present invention is applicable to commercially pure grades of titanium, titanium alloys (xcex1,xcex1+xcex2, or xcex2 alloys), commercially pure grades of zirconium, zirconium alloys and to alloys of zirconium and titanium.
Where the article is required to have good fatigue properties, it may be subjected to a mechanical surface treatment, such as shot peening, after heat treatment in order to restore the fatigue properties which may be reduced by the heat treatment operation.
According to a third aspect of the present invention, there is provided an article formed of a metal or alloy selected from titanium, zirconium, alloys of titanium and alloys of zirconium, said article having a hardened metallic case, strengthened by diffused oxygen; wherein the article has a sigmoid-shaped hardness profile across said hardened case.
Preferably, the depth of the hardened case is greater than 50 xcexcm, and is typically in the range 200 to 500 xcexcm, but may be as great as 1 mm.
A further layer of low-friction material, for example, a nitride, diamond-like-carbon or an oxide layer as described in our co-pending PCT Publication No. WO98/02595, may be provided on top of the hardened case.